Backside illumination image sensor and image-capturing device

ABSTRACT

A backside illumination image sensor that includes a semiconductor substrate with a plurality of photoelectric conversion elements and a read circuit formed on a front surface side of the semiconductor substrate, and captures an image by outputting, via the read circuit, electrical signals generated as incident light having reached a back surface side of the semiconductor substrate is received at the photoelectric conversion elements includes: a light shielding film formed on a side where incident light enters the photoelectric conversion elements, with an opening formed therein in correspondence to each photoelectric conversion element; and an on-chip lens formed at a position set apart from the light shielding film by a predetermined distance in correspondence to each photoelectric conversion element. The light shielding film and an exit pupil plane of the image forming optical system achieve a conjugate relation to each other with regard to the on-chip lens.

INCORPORATION BY REFERENCE

This is a Continuation of application Ser. No. 13/033,187 filed Feb. 23,2011 and issued as U.S. Pat. No. 8,928,795, which claims priority toJapanese Patent Application No. 2010-040378 filed Feb. 25, 2010. Thedisclosure of the prior applications is hereby incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backside illumination image sensorand an image-capturing device.

2. Description of Related Art

The performance of an image sensor at low brightness is often improvedby forming on-chip lenses, each in correspondence to one of thephotoelectric conversion elements, and illuminating each photoelectricconversion element with condensed light. The backside illumination imagesensor disclosed in Japanese Laid Open Patent Publication No.2009-164385 captures an image as signals generated with lightilluminating the back side of a semiconductor substrate, which isreceived at photoelectric conversion elements disposed in thesemiconductor substrate, are output through a read circuit formed on thefront side of the semiconductor substrate. In order to receive red-colorlight with a significant wavelength at high efficiency, photoelectricconversion elements (photodiodes) formed to assure a thickness ofapproximately 10 μm are disposed at the backside illumination imagesensor. The surface of each photoelectric conversion element and thecorresponding on-chip lens are set over a short distance from each otherand the light having passed through the on-chip lens is condensed withinthe photoelectric conversion element.

SUMMARY OF THE INVENTION

The on-chip lenses in the backside illumination image sensor in therelated art described above are designed by giving priority tocondensing performance and regulating the amount of shading in theperiphery of the image plane. However, since they are designed withoutfully taking into consideration the positional relationship between thebackside illumination image sensor and the exit pupil of the imageforming optical system, the linearity of the relationship between theaperture F number at the image forming optical system and the level ofthe signals output from the backside illumination image sensor cannot bekept intact, necessitating that correction be executed as part ofexposure control, particularly at low F numbers.

According to the present invention, the linearity representing therelationship between the aperture F number at the image forming opticalsystem and the level of the signals output by the backside illuminationimage sensor can be maintained with ease.

According to the 1st aspect of the present invention, a backsideillumination image sensor that includes a semiconductor substrate with aplurality of photoelectric conversion elements formed thereat and a readcircuit formed on a side where a front surface of the semiconductorsubstrate is present, and captures an image by outputting, via the readcircuit, electrical signals generated as incident light having passedthrough an image forming optical system and having reached a side wherea back surface of the semiconductor substrate is present, is received atthe plurality of photoelectric conversion elements comprises: a lightshielding film formed on a side where incident light enters theplurality of photoelectric conversion elements, with an opening formedtherein in correspondence to each of the plurality of photoelectricconversion elements; and an on-chip lens formed at a position set apartfrom the light shielding film by a predetermined distance incorrespondence to each of the plurality of photoelectric conversionelements. The light shielding film and an exit pupil plane at which anexit pupil of the image forming optical system is present achieve aconjugate relation to each other with regard to the on-chip lens.

According to the 2nd aspect of the present invention, in the backsideillumination image sensor according to the 1st aspect, it is preferredthat a radius of curvature R of the on-chip lens, a distance D from anapex of the on-chip lens to the light shielding film and an averagerefractive index n of a medium present between the on-chip lens and thelight shielding film achieve a relationship expressed as; D=R·n/(n−1).

According to the 3rd aspect of the present invention, in the backsideillumination image sensor according to the 2nd aspect, it is preferredthat the plurality of photoelectric conversion elements are disposed ina two-dimensional grid array; and a pitch P of the two-dimensional gridarray, a smallest F number F0 of the exit pupil of the image formingoptical system and the distance D achieve a relationship expressed as;F0·P·n>D>P·n/(2·(n−1)).

According to the 4th aspect of the present invention, in the backsideillumination image sensor according to the 3rd aspect, it is preferredthat the radius of curvature over a periphery of the on-chip lens isgreater than the radius of curvature at a central area of the on-chiplens.

According to the 5th aspect of the present invention, in the backsideillumination image sensor according to the 1st aspect, it is preferredthat the plurality of photoelectric conversion elements include a pairof focus detection photoelectric conversion elements; and the pair offocus detection photoelectric conversion elements generates a pair offocus detection signals pertaining to an image forming condition for theimage forming optical system by receiving a pair of light fluxes havingpassed through a pair of areas in the image forming optical system.

According to the 6th aspect of the present invention, in the backsideillumination image sensor according to the 5th aspect, it is preferredthat at least either a first opening formed with an offset toward oneside relative to an optical axis of the on-chip lens, or a secondopening formed with an offset toward a side opposite from the one siderelative to the optical axis of the on-chip lens, is formed as theopening in the light shielding film; and the pair of focus detectionsignals include an electrical signal generated by one of the pair offocus detection photoelectric conversion elements by receiving one lightflux in the pair of light fluxes, which passes through the firstopening, and an electrical signal generated by another focus detectionphotoelectric conversion element in the pair of focus detectionphotoelectric conversion elements by receiving another light flux in thepair of light fluxes, which passes through the second opening, and theimage forming condition for the image forming optical system can bedetected based upon a phase difference manifested by the pair of focusdetection signals.

According to the 7th aspect of the present invention, in the backsideillumination image sensor according to the 1st aspect, it is preferredthat the backside illumination image sensor further comprises: a lightshielding member that prevents entry of passing light, which is part ofthe incident light that passes through the on-chip lens, into aphotoelectric conversion element corresponding to an adjacent on-chiplens adjacent to the on-chip lens, among the plurality of photoelectricconversion elements. The light shielding member is disposed between theon-chip lens and the light shielding film.

According to the 8th aspect of the present invention, in the backsideillumination image sensor according to the 7th aspect, it is preferredthat the light shielding member is a barrier member disposed parallel toan optical axis of the on-chip lens; and an anti-reflection film isformed at a surface of the barrier member so as to prevent reflection ofthe passing light.

According to the 9th aspect of the present invention, in the backsideillumination image sensor according to the 7th aspect, it is preferredthat the light shielding member includes a first color filter disposednear the on-chip lens and a second color filter assuming a colormatching the color of the first color filter, which is disposed near theopening; and a color of the light shielding member is different from acolor of a light shielding member disposed in conjunction with theadjacent on-chip lens adjacent to the on-chip lens.

According to the 10th aspect of the present invention, in the backsideillumination image sensor according to the 1st aspect, it is preferredthat a transparent medium is filled between the on-chip lens and thelight shielding film so as to not leave any unfilled gap; and thetransparent medium is constituted of a material different from amaterial constituting the on-chip lens.

According to the 11th aspect of the present invention, in the backsideillumination image sensor according to the 10th aspect, it is preferredthat a color filter is disposed between the on-chip lens and thetransparent medium; and a wavelength range of light transmitted throughthe transparent medium includes a wavelength range of light transmittedthrough the color filter.

According to the 12th aspect of the present invention, a backsideillumination image sensor that includes a semiconductor substrate with aplurality of photoelectric conversion elements, containing a pair offocus detection photoelectric conversion elements, formed thereat and aread circuit formed on a side where a front surface of the semiconductorsubstrate is present, and captures an image by outputting, via the readcircuit, electrical signals generated as incident light having passedthrough an image forming optical system and having reached a side wherea back surface of the semiconductor substrate is present, is received atthe plurality of photoelectric conversion elements comprises: a lightshielding film formed on a side where the incident light enters theplurality of photoelectric conversion elements, with at least either afirst opening or a second opening formed therein in correspondence toeach of the plurality of photoelectric conversion elements; and anon-chip lens formed at a position set apart from the light shieldingfilm by a predetermined distance in correspondence to each of theplurality of photoelectric conversion elements. The first opening isformed with an offset toward one side relative to an optical axis of theon-chip lens; the second opening is formed with an offset toward a sideopposite from the one side relative to the optical axis of the on-chiplens; the pair of focus detection photoelectric conversion elementsgenerates a pair of focus detection signals pertaining to an imageforming condition for the image forming optical system by receiving apair of light fluxes having passed through a pair of areas in the imageforming optical system; and the pair of focus detection signals includean electrical signal generated by one of the pair of focus detectionphotoelectric conversion elements by receiving one light flux in thepair of light fluxes, which passes through the first opening, and anelectrical signal generated by another focus detection photoelectricconversion element in the pair of focus detection photoelectricconversion elements by receiving another light flux in the pair of lightfluxes, which passes through the second opening, and the image formingcondition for the image forming optical system can be detected basedupon a phase difference manifested by the pair of focus detectionsignals.

According to the 13th aspect of the present invention, animage-capturing device comprises: a backside illumination image sensoraccording to the 1st aspect; and an optical system. The optical systemis the image forming optical system that emits the incident light to beused to form an image on the backside illumination image sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a latitudinal sectional view, showing the structure of thedigital still camera achieved in an embodiment.

FIG. 2 shows focus detection positions set on the photographic imageplane of the interchangeable lens.

FIG. 3 is a front view showing the structure of the image sensor indetail.

FIG. 4 shows the shape of the on-chip lens included in animage-capturing pixel or a focus detection pixel.

FIG. 5 is a front view of an image-capturing pixel.

FIGS. 6A through 6D each show a focus detection pixel in a front view.

FIG. 7 is a sectional view of image-capturing pixels.

FIG. 8 is a sectional view of focus detection pixels.

FIG. 9 illustrates the relationship between the plane at which the lightshielding film at the image-capturing pixels is disposed and the planeat which the exit pupil of the image forming optical system is present.

FIG. 10 illustrates the relationship between the plane at which thelight shielding film the focus detection pixels is disposed and theplane at which the exit pupil of the image forming optical system ispresent.

FIG. 11 is a diagram that facilitates observation of the opticalrequirements which must be met to allow the light shielding film planeand the exit pupil plane to be conjugate with each other.

FIG. 12 is a diagram that facilitates observation of the size of animage of an aperture opening located at the exit pupil plane, which isformed on the light shielding film plane.

FIG. 13 shows light shielding members 66 disposed at the four corners ofeach on-chip lens so as to block stray light.

FIG. 14 is a sectional view of image-capturing pixels.

FIG. 15 is a sectional view of focus detection pixels.

FIG. 16 illustrates the two-dimensional positional relationship betweenthe polarizers H and the polarizers V.

FIGS. 17A and 17B each show a focus detection pixel in a front view.

FIG. 18 illustrates the two-dimensional positional relationship amongthe color filters.

FIG. 19 is a sectional view of image-capturing pixels.

FIG. 20 is a sectional view of focus detection pixels.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following is a description of a digital still camera used inconjunction with interchangeable lenses, representing an example of animage-capturing device equipped with a backside illumination sensor(hereafter simply referred to as an “image sensor”) achieved in anembodiment of the present invention is now explained. FIG. 1 is alateral sectional view of the structure adopted in the digital stillcamera in the embodiment. A digital still camera 201 achieved in theembodiment comprises an interchangeable lens 202 and a camera body 203.The interchangeable lens 202 is mounted at the camera body 203 via amount unit 204. Namely, the interchangeable lens 202 that includesvarious image forming optical systems can be mounted at the camera body203 via the mount unit 204.

The interchangeable lens 202 includes a a lens 209, a zooming lens 208,a focusing lens 210, an aperture 211, a lens drive control device 206and the like. The lens drive control device 206 is constituted with amicrocomputer, a memory, a drive control circuit and the like (noneshown). The lens control device 206 executes drive control of thefocusing lens 210 and the aperture 211 respectively for focus adjustmentand aperture opening diameter adjustment and detects the states of thezooming lens 208, the focusing lens 210 and the aperture 211. The lensdrive control device 206 also engages in communication with a body drivecontrol device 214 to be detailed later to transmit lens information tothe body drive control device and receive camera information (defocusamount, aperture value and the like) from the body drive control device.The aperture 211 forms an opening with a variable opening diameter,centered on the optical axis, so as to adjust the amount of light andadjust the extent of blurring.

An image sensor 212, the body drive control device 214, a liquid crystaldisplay element drive circuit 215, a liquid crystal display element 216,an eyepiece lens 217, a memory card 219 and the like are disposed at thecamera body 203. Image-capturing pixels are two-dimensionally arrayed inimage-capturing pixel rows and image-capturing pixel columns at theimage sensor 212, and focus detection pixels are also built into theimage sensor over areas corresponding to focus detection positions(focus detection areas). The image sensor 212 will be described indetail later.

The body drive control device 214 includes a microcomputer, a memory, adrive control circuit and the like. The body drive control device 214repeatedly executes drive control for the image sensor 212, a read ofpixel signals from the image sensor 212, a focus detection operationbased upon the pixel signals from focus detection pixels, and focusadjustment for the interchangeable lens 202. It also processes andrecords image signals and controls operations of the digital stillcamera 201. In addition, the body drive control device 214 engages incommunication with the lens drive control device 206 via an electricalcontact point 213 to receive the lens information and transmit thecamera information.

The liquid crystal display element 216 functions as an electronicviewfinder (EVF). A live view image expressed with image data read outfrom the image sensor 212, which is brought up on display at the liquidcrystal display element 216 by the liquid crystal display element drivecircuit 215, can be observed by the photographer via the eyepiece lens217. The memory card 219 is an image storage medium in which image dataexpressing an image captured by the image sensor 212 are stored.

A subject image is formed on the image sensor 212 with a light fluxhaving passed through the interchangeable lens 202. The subject imageundergoes photoelectric conversion at the image sensor 212.Subsequently, pixel signals output from the image-capturing pixels andthe focus detection pixels as a result of the photoelectric conversionare transmitted to the body drive control device 214.

The body drive control device 214 calculates the defocus amountindicating the extent of defocusing based upon pixel signals output fromthe focus detection pixels at the image sensor 212 and transmits thisdefocus amount to the lens drive control device 206. In addition, thebody drive control device 214 generates image data by processing thepixel signals originating from the image-capturing pixels at the imagesensor 212 and stores the image data thus generated into the memory card219. It also provides live view image signals read out from the imagesensor 212 to the liquid crystal display element drive circuit 215 so asto bring up a live view image on display at the liquid crystal displayelement 216. Moreover, the body drive control device 214 providesaperture control information to the lens drive control device 206 toenable opening control of the aperture 211.

The lens drive control device 206 updates the lens information incorrespondence to the current focusing state, zooming state and aperturesetting state, F number for the maximum aperture number and the like.More specifically, the lens drive control device 206 detects thepositions of the zooming lens 208 and the focusing lens 210 and theaperture value set for the aperture 211. It then calculates correct lensinformation based upon the lens positions and the aperture value.Alternatively, it may select the lens information corresponding to thelens positions and the aperture value from a lookup table prepared inadvance.

The lens drive control device 206 calculates a lens drive quantityindicating the extent to which the lens is to be driven based upon thedefocus amount having been received and drives the focusing lens 210 toa focusing position based upon the lens drive quantity. The lens drivecontrol device 206 also drives the aperture 211 in correspondence to theaperture value it has received.

Focus detection positions (focus detection areas) that may be set on thephotographic image plane of the interchangeable lens 202, at which animage is sampled on the photographic image plane for focus detection viafocus detection pixel rows at the image sensor 212 as described later(focus detection areas, focus detection positions), are shown in FIG. 2.In this example, five focus detection areas 101 through 105 are set in arectangular photographic image plane 100, one at the center (theposition corresponding to the focus detection pixels present on theoptical axis of the interchangeable lens 202), and the other four eachset on an upper side, a lower side, a left side or a right side relativeto the center. Focus detection pixels are arrayed side-by-side in astraight line along the longer side of each of the focus detection areasindicated as rectangular areas. The focus detection pixels in the focusdetection areas 101, 102 and 103 are set side-by-side along thehorizontal direction, whereas the focus detection pixels in the focusdetection areas 104 and 105 are set side-by-side along the verticaldirection.

FIG. 3 is a front view showing in detail the structure adopted in theimage sensor 212. FIG. 3 shows in detail the pixel array pattern assumedin the image sensor in an enlarged view of an area around the focusdetection area 104 or 105 in FIG. 2. The image sensor 212 includesimage-capturing pixels 310 disposed in a dense two-dimensional squaregrid array. The image-capturing pixels 310, which include red pixels(R), green pixels (G) and blue pixels (B), are disposed in compliancewith a Bayer array pattern rule. FIG. 3 shows a focus detection pixel313 and a focus detection pixel 314 (shown in front views in the FIGS.6A and 6B respectively) assuming a pixel size matching that of theimage-capturing pixels 310 and engaged in focus detection along thevertical direction, which are disposed alternately to each other in alinear iteration along the vertical direction, so as to occupysuccessive positions that would be normally occupied by green pixels andred pixels.

In the horizontal focus detection pixel array pattern assumed for thefocus detection pixels included in the focus detection areas 101, 102and 103 in FIG. 2, a focus detection pixel 315 and a focus detectionpixel 316 (shown in front views in the FIGS. 6C and 6D respectively),assuming a pixel size matching that of the image-capturing pixels 310and engaged in focus detection along the horizontal direction, aredisposed alternately to each other in a linear iteration along thehorizontal direction, so as to occupy successive positions that would benormally occupied by green pixels and blue pixels.

FIG. 4 shows the shape of the on-chip lenses 10 included in theimage-capturing pixels 310 and the focus detection pixels 313, 314, 315and 316. The on-chip lenses 10 of the image-capturing pixels 310 and thefocus detection pixels 313, 314, 315 and 316 assume a shape achieved bycutting out a square-shaped lens with the size thereof matching thepixel size from a round on-chip lens 1 larger than the pixels. The shapeof the section of the on-chip lens 10, taken along a diagonal passingthrough the optical axis, and the shape of the section of the on-chiplens 10, taken along a horizontal line passing through the optical axis,are as shown in FIG. 4.

Three types of color filters, red (R) green (G) and blue (B) colorfilters, are disposed at the image-capturing pixels 310. Each type ofcolor filter assumes spectral sensitivity characteristics correspondingto one of the three colors; red, green and blue. The image-capturingpixels 310 each equipped with a color filter corresponding to a specificcolor are disposed in a Bayer array at the image sensor 212.

FIG. 5 is a front view of an image-capturing pixel 310. As describedearlier, the on-chip lens 10 assumes a square shape. An opening 11,formed on a light shielding film to be described later, defines a squarelight receiving area for the photoelectric conversion element.

White-color filters through which all the visible light is transmittedare disposed at the focus detection pixels 313, 314, 315 and 316 so asto enable focus detection in correspondence to all the colors. Thespectral sensitivity characteristics of the white-color filters areequivalent to the sum of the spectral sensitivity characteristics of thegreen pixels, the red pixels and the blue pixels among theimage-capturing pixels 310 described earlier. The light wavelength rangecorresponding to the spectral sensitivity characteristics of thewhite-color filters contains the individual light wavelength ranges overwhich the green pixels, the red pixels and the blue pixels demonstratehigh spectral sensitivity characteristics.

FIGS. 6A, 6B, 6C and 6D respectively present front views of a focusdetection pixel 313, a focus detection pixel 314, a focus detectionpixel 315 and a focus detection pixel 316. FIG. 6A shows the focusdetection pixel 313 in a front view. Its on-chip lens 10 assumes asquare shape. An opening 13, formed at a light shielding film to bedescribed later, limits the light receiving area of the photoelectricconversion element to an upper half of a square (the upper half of asquare split into two equal portions along a horizontal line).

FIG. 6B shows the focus detection pixel 314 in a front view. Its on-chiplens 10 assumes a square shape. An opening 14, formed at a lightshielding film to be described later, limits the light receiving area ofthe photoelectric conversion element to a lower half of a square (thelower half of a square split into two equal portions along a horizontalline).

When the focus detection pixel 313 and the focus detection pixel 314 arestacked one on top of the other by aligning their on-chip lenses 10, theopenings 13 and 14 formed at the light shielding film are setside-by-side along the vertical direction and achieve a complementaryrelation to each other across a boundary set at the horizontal splittingline that splits the opening 11 at the image-capturing pixel 310 intotwo equal portions.

FIG. 6C shows the focus detection pixel 315 in a front view. Its on-chiplens 10 assumes a square shape. An opening 15, formed at a lightshielding film to be described later, limits the light receiving area ofthe photoelectric conversion element to a left half of a square (theleft half of a square split into two equal portions along a verticalline).

FIG. 6D shows the focus detection pixel 316 in a front view. Its on-chiplens 10 assumes a square shape. An opening 16, formed at a lightshielding film to be described later, limits the light receiving area ofthe photoelectric conversion element to a right half of a square (theright half of a square split into two equal portions along a verticalline).

When the focus detection pixel 315 and the focus detection pixel 316 arestacked one on top of the other by aligning their on-chip lenses 10, theopenings 15 and 16 formed at the light shielding film are setside-by-side along the horizontal direction and achieve a complementaryrelation to each other across a boundary set at the vertical splittingline that splits the opening 11 at the image-capturing pixel 310 intotwo equal portions.

FIG. 7 is a schematic sectional view of image-capturing pixels 310present near the optical axis at the backside illumination image sensorachieved in the embodiment of the present invention. As shown in FIG. 7,a photoelectric conversion element (photodiode) 404 constituted with ahigh density N-type semiconductor area is formed within each of numerousunit pixel areas 403 defined in a semiconductor substrate such as asingle crystal silicon layer 401. The photoelectric conversion elements404 in adjacent unit pixel areas 403 are separated from each other by anelement separating area 402 constituted of a P-type semiconductor formedwithin the single crystal silicon layer 401.

A light shielding film 30 is formed on one of the principal planes ofthe single crystal silicon layer 401, i.e., on a back surface 408located on the upper side in FIG. 7. Openings 11, each corresponding toone of the photoelectric conversion elements 404, are formed in thelight shielding film 30, and a transparent insulating film 31 embedseach opening 11. An opening 11 in the light shielding film 30 definesthe light receiving area of the photoelectric conversion element 404corresponding to the particular opening 11. It is to be noted that FIG.7 shows the transparent insulating film 31 also embedding the areabetween the light shielding film 30 and the back surface 408 of thesingle crystal silicon layer 401.

The light shielding film 30, constituted of a metal such as aluminum,includes an anti-reflection film vapor-deposited onto the surfacethereof. The transparent insulating film 31 is laminated so as toachieve a predetermined thickness over the light shielding film 30, anda color filter 9 corresponding to the particular photoelectricconversion element 404 is formed on the insulating film 31. Thetransparent insulating film 31 is allowed to achieve the predeterminedthickness by setting the plane at which the light shielding film 30 isdisposed in a conjugate relation with the plane of the exit pupil of theimage forming optical system. Color filters 9 include R-color filters(notated as “R” in the figure) through which light in the red colorwavelength range is transmitted, G-color filters (notated as “G” in thefigure) through which light in the green color wavelength range istransmitted and B-color filters through which light in the blue colorwavelength range is transmitted. These color filters are set in a Bayerarray.

The on-chip lens 10 corresponding to each photoelectric conversionelement 404 is formed on a color filter 9. The position of an opticalaxis 27 of the on-chip lens 10 corresponding to a given photoelectricconversion element 404 matches the position of the center of the opening11 in the light shielding film 30 corresponding to the particularphotoelectric conversion element 404. It is to be noted that in animage-capturing pixel 310 located in a peripheral area of the imagesensor 212, the position of the optical axis 27 of the on-chip lens 10and the position of the center of the opening 11 in the light shieldingfilm 30 corresponding to the photoelectric conversion element 404 in theparticular image-capturing pixel are slightly shifted relative to eachother so as to prevent shading that tends to occur readily in theperiphery of the image plane.

In addition, a barrier 8 functioning as a light shielding member rangingalong a direction parallel to the on-chip lens axis is disposed so as toextend from the light shielding film 30 to the on-chip lens 10 at theboundary between each unit pixel area 403 and an adjacent unit pixelarea. The barrier 8 is constituted of aluminum, which is used as awiring material and the like, and the surfaces of the barrier 8, i.e.,the surfaces in contact with the transparent insulating films 31, arecoated with an anti-reflection film such as titanium nitridevapor-deposited thereupon.

On the side of the single crystal silicon layer 401 where the otherprincipal plane is present, i.e., on a surface 409 located on the lowerside in FIG. 7, a read circuit 405 that reads out a signal correspondingto a signal charge having been accumulated at a specific photoelectricconversion element 404 is formed. An insulating layer 417 is formed onthe read circuit 405, with a plurality of wiring layers 418 formedinside the insulating layer 417.

The read circuit 405 includes a readout gate portion 411 constitutedwith a P-type semiconductor area, a floating diffusion portion (FDportion) 412 located adjacent to the readout gate portion 411 andconstituted with a high density N-type semiconductor area to which thesignal charge having accumulated at the photoelectric conversion element404 is transferred, a reset gate portion (not shown) via which thesignal charge having accumulated at the FD portion 412 is cleared, a MOScircuit (not shown) connected to the FD portion 412 and constituted witha MOS transistor, which outputs the signal corresponding to the signalcharge having accumulated in the FD portion 412, and a readout electrode413 formed on the readout gate portion 411.

In addition, a second element separating area 414 constituted with aP-type semiconductor area and a positive charge accumulating area 415,located adjacent to the photoelectric conversion element 404 andconstituted with a high density P-type semiconductor area formed on theside where the surface 409 of the single crystal silicon layer 401 ispresent, are formed in the single crystal silicon layer.

The wiring layers 418 include four layers of wirings. More specifically,the wiring layers 418 include a first-layer wiring 481, a second-layerwiring 482 formed above the first layer wiring 481 via the insulatinglayer 417, a third-layer wiring 483 formed above the second layer wiring482 via the insulating layer 417 and a fourth-layer wiring 484 formedabove the third layer wiring 483 via the insulating layer 417, allpresent within the insulating layer 417, which is formed on the singlecrystal silicon layer 401 on the side where the surface 409 is present.It is to be noted that a flattening film constituted with a passivationfilm (not shown) is formed upon the insulating layer 417, and asupporting substrate 416 is bonded onto the flattening film via anadhesive layer.

As will be explained later, the plane at which the light shielding film30 is disposed in an image-capturing pixel 310 of the backsideillumination image sensor structured as described above, is in aconjugate relation with the plane at which the exit pupil of the imageforming optical system that forms an image on the image sensor ispresent, with regard to the on-chip lens 10. Light 28 having entered theon-chip lens 10 located on the side where the back surface 408 of thesingle crystal silicon layer 401 is present passes through the colorfilter 9 and achieves focus at the plane at which the light shieldingfilm 30 is disposed. The light 28 restricted at the opening 11 in thelight shielding film 30 is ultimately guided to the photoelectricconversion element 404. A signal charge generated and accumulated at thephotoelectric conversion element 404 in correspondence to the light 28is transferred to the FD portion 412 through the readout gate portion411 and is accumulated in the FD portion 412 as a high voltage isapplied to the readout electrode 413. A signal corresponding to thesignal charge accumulated in the FD portion 412 is then output by theMOS circuit. Once the signal has been output, the signal charge havingaccumulated in the FD portion 412 is reset and the next exposure sessionstarts.

Stray light 21 with a large angle of incidence is blocked by the barrier8 and thus does not enter the adjacent unit pixel area 403. In addition,the anti-reflection film vapor-deposited on the surface of the barrier 8prevents the stray light 21, having been blocked at the barrier 8, frombeing reflected off the barrier 8 and entering the opening 11 in thelight shielding film 30.

The distance from the on-chip lens 10 to the light shielding film 30 inthe image sensor adopting the structure described above is greater thanthe distance between the on-chip lens and the light shielding film in apixel at a backside illumination image sensor in the related art.Without the barrier 8, even oblique light with a relatively small angleof incidence compared to the stray light 21 would be allowed to readilyenter the adjacent unit pixel area 403. In other words, the presence ofa light shielding member such as the barrier 8 is crucial. In addition,since the anti-reflection film vapor-deposited on the surface of thebarrier 8 prevents entry of the stray light 21 that would otherwise bereflected off the barrier 8, pass through the opening 11 in the lightshielding film 30 and enter the photoelectric conversion element 401,into the photoelectric conversion element 404, the quality of the signalis improved.

FIG. 8 is a schematic sectional view of focus detection pixels 313 and314 in the backside illumination image sensor achieved in the embodimentof the present invention. The focus detection pixels 313 and 314 in FIG.8 differ from the image-capturing pixels 310 only in their portionsabove the light shielding film 30, and accordingly, the followingdescription focuses on these portions.

A light shielding film 30 is formed on one of the principal planes ofthe single crystal silicon layer 401, i.e., on the back surface 408 onthe upper side in FIG. 8. An opening is formed in correspondence to eachphotoelectric conversion element 404 in the shielding film 30. Namely,an opening 13 is formed at each focus detection pixel 313 and an opening14 is formed at each focus detection pixel 314, with a transparentinsulating film 31 embedding the openings 13 and 14. The openings 13 and14 each assume a shape that is achieved by covering either the left halfor the right half of the opening 11 at an image-capturing pixel 310. Theopenings 13 and 14 formed in the light shielding film 30 define thelight receiving areas of the photoelectric conversion elements 404corresponding to the openings 13 and 14. It is to be noted that FIG. 8shows the transparent insulating film 31 also filling the area betweenthe light shielding film 30 and the back surface 408 of the singlecrystal silicon layer 401.

The light shielding film 30, constituted of a metal such as aluminum,includes an anti-reflection film vapor-deposited onto the surfacesthereof. The transparent insulating film 31 is laminated so as toachieve a predetermined thickness on the light shielding film 30, and awhite-color filter 7 is formed on the insulating film 31 incorrespondence to each photoelectric conversion element 404. Thetransparent insulating film 31 is allowed to achieve the predeterminedthickness by setting the plane at which the light shielding film 30 isdisposed is in a conjugate relation with the plane of the exit pupil ofthe image forming optical system. The on-chip lens 10 corresponding toeach photoelectric conversion element 404 is formed on a white-colorfilter 7.

In addition, a barrier 8 functioning as a light shielding member rangingalong a direction parallel to the on-chip lens axis is disposed so as toextend from the light shielding film 30 to the on-chip lens 10 at theboundary between each unit pixel area 404 and an adjacent unit pixelarea. The barrier 8 is constituted of aluminum, which is used as awiring material and the like, and the surfaces of the barrier 8, i.e.,the surfaces in contact with the transparent insulating films 31, arecoated with an anti-reflection film such as titanium nitridevapor-deposited thereupon. Light 29 having entered the on-chip lens 10located on the side where the back surface 408 of the single crystalsilicon layer 401 is present at a focus detection pixel 313 or 314 ofthe backside illumination image sensor structured as described abovepasses through the white-color filter 7 and achieves focus at the planeat which the light shielding film 30 is disposed. The light 29restricted at the opening 13 or 14 in the light shielding film 30 isultimately guided to the corresponding photoelectric conversion element404.

Stray light 21 with a large angle of incidence is blocked by the barrier8 and thus does not enter the adjacent unit pixel area 403. In addition,the anti-reflection film vapor-deposited on the surface of the barrier 8prevents the stray light 21, having been blocked at the barrier 8, frombeing reflected off the barrier 8 and entering the opening 13 or 14 inthe light shielding film 30.

As will be detailed later, it is an essential structural requirement forthe focus detection pixels 313 and 314 that they include complementaryopenings 13 and 14 formed in the light shielding film 30 and that focusbe achieved via the on-chip lenses 10 on the light shielding film 30.This means that the distance from each on-chip lens 10 to the lightshielding film 30 is bound to be greater than the distance between theon-chip lens and the light shielding film at a pixel in a backsideillumination image sensor in the related art. Without the barrier 8,even oblique light with a relatively small angle of incidence comparedto the stray light 21 would be allowed to readily enter the adjacentunit pixel area 403. In other words, the presence of a light shieldingmember such as the barrier 8 is crucial. In addition, since theanti-reflection film vapor-deposited on the surface of the barrier 8prevents entry of the stray light 21 that would otherwise be reflectedoff the barrier wall 8, pass through the opening 13 or 14 in the lightshielding film 30 and enter the photoelectric conversion element 401,into the photoelectric conversion element 404, the quality of the signalis improved and the focus detection accuracy is thus ultimatelyimproved.

The structures of the focus detection pixels 315 and 316 are basicallyidentical to the structures of the focus detection pixels 313 and 314shown in FIG. 8, except for their orientations, as they are rotated by90° relative to the focus detection pixels 313 and 314.

FIG. 9 shows the relationship between the plane at which the lightshielding film 30 in the image-capturing pixels 310 is disposed and theplane at which the exit pupil of the image forming optical system ispresent. The positions of the openings 11 formed in the light shieldingfilm 30 in correspondence to all the image-capturing pixels 310 arrayedon the image sensor 212 each achieve a conjugate relation with theposition of a common area 95 defined by all the image-capturing pixels310 on an exit pupil plane 90, which is set apart from the on-chiplenses 10 by a pupil distance L, with regard to the on-chip lens 10 inthe corresponding image-capturing pixel 310. Namely, an image formingrelationship exists whereby the plane at which the light shielding film30 is disposed is in a conjugate relation with the exit pupil plane 90with regard to the image forming plane, i.e., the apex plane at whichthe apex of the on-chip lens 10 is set and a signal corresponding to theintensity of an image formed on the image forming plane is output fromthe corresponding image-capturing pixel 310. Under these circumstances,as the openings 11 formed in the light shielding film 30 incorrespondence to the individual image-capturing pixels 310 areprojected via the on-chip lenses 10 onto the exit pupil plane 10, theprojection image of the opening 11 of each image-capturing pixel 310invariably defines the area 95.

Accordingly, each image-capturing pixel 310 receives a light flux 71having passed through the area 95 and the on-chip lens 10 at theparticular image-capturing pixel 310, and then outputs a signalcorresponding to the intensity of the image formed with the light flux71 on the on-chip lens 10.

It is to be noted that the positional relationship between the positionof the optical axis of the on-chip lens 10 and the center of the opening11 is shifted for each image-capturing pixel 310 in correspondence tothe image height of the particular image-capturing pixel 310, so as toensure that the light flux 71 having passed through the common area 95shared by all the image-capturing pixels, defined on the exit pupilplane 90, is received at the image-capturing pixel 310 regardless of itsimage height.

As described above, since the plane at which the light shielding film 30is disposed is in a conjugate relation with the exit pupil plane 90, theimage-capturing pixels 310 are each allowed to receive light in anamount in proportion to the area of the aperture opening on the exitpupil plane 90 and thus, better linearity is assured in the exposurecontrol.

While the position of the exit pupil plane 90 varies among differentinterchangeable lenses 202 used in an interchangeable lens system, thepupil distance L may, in practice, be set based upon the average exitpupil plane position assumed in the interchangeable lens system.

FIG. 10 shows the relationship between the plane at which the lightshielding film 30 at the focus detection pixels 313 and 314 is disposedand the plane at which the exit pupil of the image forming opticalsystem is present.

The positions of the openings 13 formed in the light shielding film 30in correspondence to all the focus detection pixels 313 arrayed on theimage sensor 212 each achieve a conjugate relation with the position ofa common area 93 defined by all the focus detection pixels on the exitpupil plane 90, which is set apart from the on-chip lenses 10 by thepupil distance L, with regard to the on-chip lens 10 in thecorresponding focus detection pixel 313. Namely, an image formingrelationship exists whereby the plane at which the light shielding film30 is disposed is in a conjugate relation with the exit pupil plane 90with regard to the image forming plane, i.e., the apex plane at whichthe apex of the on-chip lens 10 is set and a signal corresponding to theintensity of an image formed on the image forming plane is output fromthe corresponding focus detection pixel 313. Under these circumstances,as the openings 13 formed in the light shielding film 30 incorrespondence to the individual focus detection pixels 313 areprojected via the on-chip lenses 10 onto the exit pupil plane 10, theprojection image of the opening 13 of each focus detection pixel 313invariably defines the area 93.

In addition, the positions of the openings 14 formed in the lightshielding film 30 in correspondence to all the focus detection pixels314 arrayed on the image sensor 212 each achieve a conjugate relationwith the position of a common area 94 defined by all the focus detectionpixels on an exit pupil plane 90, which is set apart from the on-chiplenses 10 by the pupil distance L, with regard to the on-chip lens 10 inthe corresponding focus detection pixel 314. Namely, an image formingrelationship exists whereby the plane at which the light shielding film30 is disposed is in a conjugate relation with the exit pupil plane 90with regard to the image forming plane, i.e., the apex plane at whichthe apex of the on-chip lens 10 is set and a signal corresponding to theintensity of an image formed on the image forming plane is output fromthe corresponding focus detection pixel 314. Under these circumstances,as the openings 14 formed in the light shielding film 30 incorrespondence to the individual focus detection pixels 314 areprojected via the on-chip lenses 10 onto the exit pupil plane 10, theprojection image of the opening 14 of each focus detection pixel 314invariably defines the area 94.

In the description given in reference to FIG. 10, the pair of areas 93and 94 are referred to as a pair of focus detection pupils 93 and 94.Each focus detection pixel 313 receives a light flux 73 having passedthrough the focus detection pupil 93 and the on-chip lens 10 at theparticular focus detection pixel 313 and outputs a signal correspondingto the intensity of the image formed with the light flux 73 on theon-chip lens 10. Each focus detection pixel 314 receives a light flux 74having passed through the focus detection pupil 94 and the on-chip lens10 at the particular focus detection pixel 314 and outputs a signalcorresponding to the intensity of the image formed with the light flux74 on the on-chip lens 10.

A combined area made up with the focus detection pupils 93 and 94 on theexit pupil 90, through which the light fluxes 73 and 74 to be receivedat each pair of focus detection pixels 313 and 314 pass, matches thearea 95 on the exit pupil 90, through which the light fluxes 71 to bereceived at the image-capturing pixels 310 pass. The light fluxes 73 and74 assume a complementary relationship to each other on the exit pupil90 in relation to the light fluxes 71.

Numerous focus detection pixels 313 and 314 structured as describedabove, are disposed so that a focus detection pixel 313 and a focusdetection pixel 314 paired up with the focus detection pixel 313 arearrayed alternately to each other in a linear iteration. By integratingthe outputs from the photoelectric conversion elements 404 of theindividual focus detection pixels 313 and 314 into a pair of outputgroups, one corresponding to the focus detection pupil 93 and the othercorresponding to the focus detection pupil 94, information pertaining tothe intensity distributions of the pairs of images formed on the focusdetection pixel row (extending along the vertical direction) by thepairs of light fluxes passing through the focus detection pupil 93 andthe focus detection pupil 94 is obtained. Then, image shift detectionoperation processing (correlation calculation processing, phasedifference detection processing) of the known art is executed inconjunction with this information, so as to detect an image shift amountindicating the extent of image shift manifested by the image pairsthrough a method often referred to as the split pupil phase detectionmethod. Then, the image shift amount undergoes conversion calculationexecuted in correspondence to a proportional relation of the distancebetween the gravitational centers of the pair of focus detection pupilsto the focus detection pupil distance, so as to determine the extent ofdeviation (defocus amount) of the actual image forming plane relative tothe predetermined image forming plane at the focus detection position(along the vertical direction).

The focus detection light fluxes received at each pair of focusdetection pixels 315 and 316 are basically identical to the light fluxesshown in FIG. 10 except for their orientations, which are rotated by 90°relative to the orientations of the pair of focus detection pixels 73and 74 received at the corresponding focus detection pixels 313 and 314.A pair of focus detection pupils, rotated by 90° relative to the focusdetection pixels 93 and 94, is set in correspondence to the pairs offocus detection pixels 315 and 316. Numerous focus detection pixels 315and 316, are disposed so that a focus detection pixel 315 and a focusdetection pixel 316 paired up with the focus detection pixel 315 arearrayed alternately to each other in a linear iteration. By integratingthe outputs from the photoelectric conversion elements 404 of theindividual focus detection pixels 315 and 316 into a pair of outputgroups, each corresponding to one of the focus detection pupils pairedup with each other, information pertaining to the intensitydistributions of the pairs of images formed on the focus detection pixelrow (extending along the horizontal direction) by the pairs of lightfluxes passing through the pair of focus detection pupils is obtained.The extent of deviation (defocus amount) of the actual image formingplane relative to the predetermined image forming plane at the focusdetection position (along the horizontal direction) can be calculatedbased upon the information thus obtained.

FIG. 11 facilitates observation of the optical requirements that must befulfilled to allow the plane at which the light shielding film 30 isdisposed and the exit pupil plane 90 to achieve a conjugate relationwith each other with a simplified illustration of the optical system ina unit pixel area 403 where an image-capturing pixel or a focusdetection pixel is formed.

It is desirable, under normal circumstances, to achieve a soundconjugate relation by using non-spherical on-chip lenses 10. Thefollowing description, given by assuming that the on-chip lensesdisposed in the image sensor, which are extremely small, assume aspherical lens contour with a constant radius of curvature, can also beapplied to non-spherical on-chip lenses 10 through optimalapproximation.

R represents the radius of curvature of an on-chip lens 10, L representsthe distance between the apex T of the on-chip lens 10 to the exit pupilplane 90, D represents the distance from the apex T of the on-chip lens10 to the plane at which the light shielding film 30 is disposed, n0represents the average refractive index of the medium present betweenthe on-chip lens 10 and the exit pupil plane 90, and n represents theaverage refractive index for the on-chip lens 10 and the medium presentbetween the on-chip lens 10 and the light shielding film 30.

A condition expression defining requirements for allowing the plane atwhich the light shielding film 30 is disposed and the exit pupil plane90 to achieve a conjugate relation with regard to the on-chip lens 10 isdetermined. The condition defined in (1) below must be satisfied for thedistance D between the apex T of the on-chip lens 10 and the plane atwhich the light shielding film 30 is disposed, determined throughapproximation; distance L>>distance D, based upon a geometric-opticsparaxial imaging condition.D=R·n/(n−n0)  (1)Expression (2) below is obtained by assuming that the medium presentbetween the on-chip lens 10 and the exit pupil plane 90 is air, i.e.,refractive index n0=1, in a further approximation.D=R·n/(n−1)  (2)

The condition as defined in (3) is ascertained by modifying expression(2) on the premise that the radius of curvature R is greater than halfthe size of the unit pixel area 403, i.e., half the pixel pitch P. Thepremise that the radius of curvature R is greater than half the pixelpitch P is set forth so as to increase the light receiving efficiency byeliminating any dead zone in the photoelectric conversion element 404where no light is received. It is to be noted that the curvature at theperiphery of the on-chip lens 10 should be set more gradual compared tothe curvature at the center of the on-chip lens 10 in consideration ofspherical aberration, as will be described later.D>P·n/(2·(n−1))  (3)

In addition, while the on-chip lens 10 assumes a spherical contour,focus is achieved with a light beam 62 passing through the periphery ofthe on-chip lens 10 at a position closer to the on-chip lens 10 due tothe spherical aberration, relative to the focusing position achievedwith a light beam 61 passing through an area near the center of theon-chip lens 10. Accordingly, the radius of curvature of the on-chiplens 10 is adjusted for purposes of spherical aberration correction sothat it gradually increases with the extent of the increase becominggreater further toward the periphery of the on-chip lens 10 relative tothe radius of curvature assumed at the center of the on-chip lens 10.Through these measures, the conjugate relation between the plane atwhich the light shielding film 30 is disposed and the exit pupil plane90 is sustained more rigorously, which, in turn, makes it possible toimprove the focus detection accuracy and improve the linearity of theexposure control.

FIG. 12 indicates the size of the image of an aperture opening 96 set onthe exit pupil plane 90, which is formed on the plane at which the lightshielding film 30 is disposed. When the on-chip lens 10 assumes aspherical lens contour with the radius of curvature R, a light beam 63having exited an edge of the aperture opening 96 to travel toward thecurvature center 65 of the sphere with the radius of curvature R doesnot become refracted at the center of the spherical surface of the lenswith the radius of curvature R. By geometrically graphing the light beam63 traveling from the edge of the aperture opening 96 with a diameter Qtoward the curvature center 65, a diameter S of the image of theaperture opening 96 formed on the plane at which the light shieldingfilm 30 is disposed can be determined as expressed in (4) below.S=(Q·(D−R))/(L+R)  (4)

Expression (4) can be modified to expression (5) below with Frepresenting the F number of the aperture opening, which is expressed asF=L/Q, through an approximation expressed as; distance L>>radius ofcurvature R.S=(D−R)/F  (5)

It is assumed that P represents the pixel pitch and that the smallest Fnumber that is valid in the interchangeable lens system is the brightestF number, i.e., F0. As long as the light beam having entered the unitpixel area does not enter an adjacent unit pixel area (as long as P isgreater than S), the relationship expressed in expression (6) below,obtained by using expression (2) for substitution with regard to theradius of curvature R and simplifying the resulting expression for D,exists.D<F0·P·n  (6)

Accordingly, the distance D from the apex T of the on-chip lens 10 tothe plane at which the light shielding film 30 is disposed must bedetermined so as to satisfy the relationships defined in expressions (3)and (6). For instance, the average refractive index n for the on-chiplens 10 and the medium present between the on-chip lens 10 and the lightshielding film 30, the pixel pitch P and the smallest F number F0 may berespectively 1.5, 4 μm and 1.4. In this situation, as long as thedistance D from the apex T of the on-chip lens 10 to the plane at whichthe light shielding film 30 is disposed is set within a range of 6 μm to8.4 μm, a robust conjugate relation between the exit pupil plane 90 andthe plane at which the light shielding film 30 is disposed is assuredand entry of stray light into the adjacent pixels can be effectivelyprevented.

Expressions (1) through (6) can be used even when a plurality ofdifferent types of media with varying refractive indices are presentbetween the on-chip lens 10 and the light shielding film 30. Namely,under such circumstances, expressions (1) through (6) can be used byassuming an average refractive index for the on-chip lens 10 and themedia present between the on-chip lens 10 and the light shielding film30. In addition, expressions (1) through (6) can be used even when theon-chip lens 10 is constituted with a plurality of lenses, e.g., evenwhen an inner lens is added, by approximating the lens function as thatof a single lens.

As described earlier, while the curvature is set more gradual at theperiphery of the on-chip lens 10, stray light attributable to multiplereflection, which tends to occur readily at the four corners of thepixel located at the ends of the diagonals where the lens surface slopesmore acutely. FIG. 13 shows light shielding members 66 disposed at thefour corners of each on-chip lens in order to prevent such stray light.The light shielding members 66 may be light absorbing members fillingthe valleys in the on-chip lens array made up with the plurality ofon-chip lenses 10, or they may be light absorbing members disposed atpositions corresponding to those of the color filters 9 in FIG. 7 andthe white-color filters 7 in FIG. 8. The light absorbing members may beconstituted with, for instance, black-color filters.

In the embodiment described above, a plane conjugate with the exit pupilplane 90 of the image forming optical system, relative to the on-chiplens 10, can be set further frontward relative to the photoelectricconversion element 404 and light entering the photoelectric conversionelement 404 is restricted at the opening 11, 13 or 14 located at theconjugate plane. As a result, the linear relationship between theaperture F number and the signal level can be kept intact with ease andunnecessary entry of oblique light into an adjacent pixel can also beprevented with ease.

The structure adopted in a backside illumination image sensor in therelated art, in which the plane conjugate with the pupil is set withinthe photoelectric conversion elements, is not compatible with focusdetection pixels engaged in focus detection through the split pupilphase detection method, since openings for splitting the pupil cannot beformed at the plane. In contrast, according to the present inventiondescribed above in reference to the embodiment, the plane conjugate withthe exit pupil plane 90 of the image forming optical system relative tothe on-chip lenses 10 can be positioned further frontward relative tothe photoelectric conversion elements 404, which makes it possible toform the openings 11, 13 and 14 for splitting the pupil at the conjugateplane. Furthermore, by setting the distance D between the on-chip lenses10 and the light shielding film 30 so as to satisfy specificrequirements, focus detection pixels to be engaged in focus detectionthrough the split pupil phase detection method, which are compatiblewith a backside illumination image sensor, and also assure a high levelof accuracy, can be achieved while, at the same time, minimizing theadverse effects of stray light.

Other Embodiments of the Invention (1) Embodiment in which Entry ofLight into Adjacent Pixels is Prevented Via Polarizers

FIGS. 7 and 8 show the barriers 8, each disposed at the boundary of aunit pixel area 403 to function as a light shielding member rangingalong the direction parallel to the on-chip lens axis and extending fromthe light shielding film 30 to the on-chip lens 10. The entry of lightinto adjacent unit pixel areas 403 may be prevented by adoptingalternative measures.

FIGS. 14 through 16 present an example of light shielding measures takenby utilizing polarizers. FIG. 14 is a schematic sectional view ofimage-capturing pixels 310 disposed near the optical axis of the imageforming optical system in a backside illumination image sensor achievedin the embodiment, in which the entry of light into adjacent pixels isprevented by polarizers. Since the structural elements disposed atpositions lower than the light shielding film 30 among the structuralelements in FIG. 14 are identical to those in FIG. 7, their explanationis not provided. Polarizers H32 and polarizers V33, to function as lightshielding members, are disposed at the image-capturing pixels 310 shownin FIG. 14, in place of the barriers 8 shown in FIG. 7. A transparentinsulating film 31 embeds the polarizers H32 and V33 located directlyabove the light shielding film 30 and the color filters 9. In addition,the transparent insulating film 31 also embeds the space between thelight shielding film 30 and the polarizers H32 and V33 and the spacebetween the light shielding film 30 and the back surface 408 of thesingle crystal silicon layer 401.

The polarizers H32 and the polarizers V33, which may be, for instance,photonic crystal polarizers configured in an array such as thosedisclosed in International Publication No. 2004/008196, polarize lightalong polarizing directions perpendicular to each other. The polarizersH32 and V33 are formed on a substrate with cyclical columns of minutegrooves formed thereupon by alternately laminating a material with ahigh refractive index such as Si or Ta and a material with a lowrefractive index such as SiO₂ over multiple layers upon the substrate,with a reiterating pattern of indentations/projections formed in eachlayer.

FIG. 14 shows two polarizers H32, one disposed between the on-chip lens10 of the right-hand side image-capturing pixel 310 and thecorresponding color filter 9 and the other disposed at a positiondirectly in front of the light shielding film 30. It also shows twopolarizers V33, one disposed between the on-chip lens 10 of theleft-hand side image-capturing pixel 310 and the corresponding colorfilter 9 and the other disposed at a position directly in front of thelight shielding film 30.

Stray light 21 with a large angle of incidence will have to pass throughone polarizer V33 and one polarizer H32 assuming polarizing directionsperpendicular to each other before it ever reaches the opening 11 of theadjacent unit pixel area 403. Since the entry of the stray light 21 intothe opening 11 in the adjacent unit pixel area 403 is thus prevented,color mixture (crosstalk) does not occur. As a result, an image of highquality can be generated.

FIG. 15 is a schematic sectional view of focus detection pixels 313 and314 disposed near the optical axis of the image forming optical systemin the backside illumination image sensor achieved in the embodiment, inwhich the entry of light into adjacent pixels is prevented bypolarizers. Since the structural elements disposed at positions in thearea lower than the light shielding film 30 among the structuralelements in FIG. 15 are identical to those in FIG. 7, their explanationis not provided. Polarizers H32 and polarizers V33, to function as lightshielding members, are disposed at the focus detection pixels 313 and314 shown in FIG. 15, in place of the barriers 8 shown in FIG. 8. Atransparent insulating film 31 embeds the polarizers H32 and V33 locateddirectly above the light shielding film 30 and the white-color filters7. In addition, the transparent insulating film 31 also embeds the spacebetween the light shielding film 30 and the polarizers H32 and V33 andthe space between the light shielding film 30 and the back surface 408of the single crystal silicon layer 401.

FIG. 15 shows two polarizers H32, one disposed between the on-chip lens10 of the focus detection pixel 314 and the white-color filter 7 and theother disposed at a position directly in front of the light shieldingfilm 30. It also shows two polarizers V33, one disposed between theon-chip lens 10 of the focus detection pixel 313 and the white-colorfilter 7 and the other disposed at a position directly in front of thelight shielding film 30.

Stray light 21 with a large angle of incidence will have to pass throughone polarizer V33 and one polarizer H32 assuming polarizing directionsperpendicular to each other before it ever reaches the opening 13 or 14of the adjacent unit pixel area 403. Since the entry of the stray light21 into the opening 13 or 14 in the adjacent unit pixel area 403 is thusprevented, color mixture (crosstalk) does not occur. As a result, highlyaccurate focus detection can be achieved.

The structures of the focus detection pixels 315 and 316 are basicallyidentical to the structures of the focus detection pixels 313 and 314shown in FIG. 15, except for their orientations, since they are rotatedby 90° relative to the focus detection pixels 313 and 314.

FIG. 16, which corresponds to FIG. 3, indicates the two-dimensionalpositional relationship among the polarizers H32 and the polarizers V33shown in FIGS. 14 and 15. With the polarizers H32 and the polarizers V33disposed in an alternate checkered pattern, entry of stray light frompixels located above, below, to the left and to the right can beeffectively prevented.

FIGS. 14 and 15 show that each unit pixel area 403 includes polarizersdisposed at two positions therein. As a structural alternative, thespace between the color filter 9 or the white-color filter 7 and thelight shielding film 30, filled with the transparent insulating film 31in the example presented in FIGS. 14 and 15, may instead be entirelytaken up by the polarizer which is disposed at the position immediatelyin front of the light shielding film 30 in the example presented inFIGS. 14 and 15. Similar advantages are achieved by adopting thealternative structure as well.

(2) Embodiment with a Pair of Light Receiving Areas Formed in Each FocusDetection Pixel

In the image sensor 212 shown in a partial enlargement in FIG. 3, focusdetection pixels 313 and 314, each equipped with a single photoelectricconversion element, are disposed so that each focus detection pixel 313is paired up with an adjacent focus detection pixel 314. However, thepresent invention may be adopted in conjunction with focus detectionpixels each equipped with a pair of photoelectric conversion elements,such as focus detection pixels 311 and 312 shown in front views in FIGS.17A and 17B.

The focus detection pixel 311 shown in FIG. 17A fulfills the functionsachieved by the pair of focus detection pixels 313 and 314 respectivelyshown in FIGS. 6A and 6B, whereas the focus detection pixel 312 shown inFIG. 17B fulfills the functions achieved by the pair of focus detectionpixels 315 and 316 respectively shown in FIGS. 6C and 6D. The focusdetection pixel 311 in FIG. 17A includes an on-chip lens 10 and a pairof light receiving areas 23 and 24, whereas the focus detection pixel312 in FIG. 17B includes an on-chip lens 10 and a pair of lightreceiving areas 25 and 26.

FIG. 18 shows the two-dimensional positional relationship among thecolor filters in an image sensor distinguishable from that shown in FIG.3 in that focus detection pixels 311 are disposed in place of the focusdetection pixels 313 and 314. FIG. 18 indicates that the focus detectionpixels 311 or 312, too, include color filters disposed in the colorfilter array pattern matching that of the image-capturing pixels 310.The two-dimensional positional relationship among the color filtersshown in FIG. 18 is also assumed in an area of the image sensor wherefocus detection pixels 312 are disposed in place of the focus detectionpixels 315 and 316.

FIG. 19 is a schematic sectional view of image-capturing pixels 310disposed near the optical axis of the image forming optical system inthe backside illumination image sensor achieved in the embodiment, whichincludes a pair of light receiving areas formed at each focus detectionpixel. Since the structural elements disposed at positions lower thanthe light shielding film 30 among the structural elements in FIG. 19 areidentical to those in FIG. 7, their explanation is not provided. As analternative to the barriers 8 shown in FIG. 7, the image-capturingpixels 310 in FIG. 19 each include a light shielding member constitutedwith another color filter 9 disposed at a position immediately in frontof the light shielding film 30. A transparent insulating film 31 fillsthe space between the color filter 9 directly above the light shieldingfilm 30 and the color filter 9 located directly below the on-chip lens10. The transparent insulating film 31 also embeds the space between thecolor filter 9 directly above the light shielding film 30 and the lightshielding film 30 itself and the space between the light shielding film30 and the back surface 408 of the single crystal silicon layer 401.However, the color filter 9 may be formed directly on the lightshielding film 30 without a transparent insulating film 31 embedding thespace above the light shielding film 30.

In the image sensor structured as described above, stray light 21 with alarge angle of incidence is bound to pass through two different types ofcolor filters 9, before it ever reaches the opening 11 in an adjacentunit pixel area 403. As explained earlier, two different types of colorfilters 9 achieve spectral sensitivity characteristics different fromeach other. For this reason, the light will have been fully attenuatedby the time it reaches the opening 11 of the adjacent unit pixel area403, and since color mixture (crosstalk) is thus prevented, an image ofhigh quality can be generated.

FIG. 20 is a schematic sectional view of focus detection pixels 311 inthe backside illumination image sensor achieved in the embodiment, whichincludes a pair of light receiving areas formed at each focus detectionpixel. As an alternative to the barriers 8 shown in FIG. 8, the focusdetection pixels 311 in FIG. 20 each include light shielding membersconstituted with two color filters 9, one disposed at a positiondirectly behind the on-chip lens 10 and the other disposed at a positiondirectly in front of an opening 11 in the light shielding film 30. Atransparent insulating film 31 fills the space between the color filter9 directly above the light shielding film 30 and the color filter 9located directly below the on-chip lens 10. The transparent insulatingfilm 31 also embeds the space between the color filter 9 directly abovethe light shielding film 30 and the light shielding film 30 itself andthe space between the light shielding film 30 and the back surface 408of the single crystal silicon layer 401. However, the color filter 9 maybe formed directly on the light shielding film 30 without a transparentinsulating film 31 embedding the space above the light shielding film30.

In the image sensor structured as described above, stray light 21 with alarge angle of incidence is bound to pass through two different types ofcolor filters 9, before it ever reaches the opening 11 in an adjacentunit pixel area 403. As explained earlier, two different types of colorfilters 9 achieve spectral sensitivity characteristics different fromeach other. For this reason, the light will have been fully attenuatedby the time it reaches the opening 11 of the adjacent unit pixel area403, and since color mixture (crosstalk) is thus prevented, highlyaccurate focus detection is enabled.

Immediately to the rear of the opening 11 in the unit pixel area 403, apair of photoelectric conversion elements 43 and 44, partitioned fromeach other by an element separating area 402 constituted with a P-typesemiconductor, are disposed. On the side where the other principal planeof the single crystal silicon layer 401 is present, i.e., on the lowerside where the front surface 409 is present in FIG. 20, a read circuit405 via which signals from the pair of photoelectric conversion elements43 and 44 are read out and wiring layers 418 are formed, together with asecond element separating area 414, a positive charge storage area 415,a support base 416 and an insulating layer 417.

In the structure described above, the boundary between the lightreceiving areas 23 and 24 shown in FIG. 17A is formed with an elementseparating area 402, whereas the outer perimeter of the light receivingareas 23 and 24 is constituted with the opening 11.

The structure of the focus detection pixels 312 is basically identicalto the structure of the focus detection pixels 311 shown in FIG. 20,except for their orientations, since they are rotated by 90° relative tothe focus detection pixel 311.

While a color filter 9 is disposed at a position immediately to the rearof the on-chip lens 10 in each unit pixel area 403 in the FIGS. 19 and20, similar advantages may be achieved by forming the on-chip lens 10itself as a color filter.

In the example presented in FIGS. 19 and 20, color filters 9 aredisposed at two positions in each unit pixel area 403. As a structuralalternative, the entire space ranging from the on-chip lens 10 to thelight shielding film 30 may be taken up by a single color filter withoutany transparent insulating film 31 embedding the space and similaradvantages will be achieved in an image sensor adopting the alternativestructure as well.

At the image sensor structured as described above, G-colorcomponent-based focus detection can be executed through phase comparison(image shift detection) of signals from the photoelectric conversionelements 43 at focus detection pixels 311 or 312 equipped with G-colorfilters and signals from the photoelectric conversion elements 44 at thefocus detection pixels 311 or 312 equipped with G-color filters. Inaddition, R-color component-based focus detection can be executedthrough phase comparison (image shift detection) of signals from thephotoelectric conversion elements 43 at focus detection pixels 311 or312 equipped with R-color filters and signals from the photoelectricconversion elements 44 at the focus detection pixels 311 or 312 equippedwith R-color filters.

At each focus detection pixel 311 or 312 equipped with a G-color filter,an image-capturing signal equivalent to an image-capturing signal thatwould be output at an image-capturing pixel 310 equipped with a G-colorfilter taking up the particular focus detection pixel position, can begenerated by adding the signal from the photoelectric conversionelements 43 and the signal from the photoelectric conversion element 44.In addition, at each focus detection pixel 311 or 312 equipped with anR-color filter, an image-capturing signal equivalent to animage-capturing signal that would be output at an image-capturing pixel310 equipped with an R-color filter taking up the particular focusdetection pixel position, can be generated by adding the signal from thephotoelectric conversion elements 43 and the signal from thephotoelectric conversion element 44.

Since an image-capturing signal can be generated with a high level ofaccuracy in correspondence to each focus detection pixel position inthis manner, better quality is assured for the captured image.

It is to be noted that the signal value representing the sum of thesignals from the pair of photoelectric conversion elements 43 and 44 maybe obtained by allowing the focus detection pixel 311 or 312 to output asum signal indicating the sum of the signals from the pair ofphotoelectric conversion elements 43 and 44 added together by an addingcircuit. As an alternative, the signals from the photoelectricconversion element 43 and the photoelectric conversion element 44 may beread out separately and then added together in an external circuit so asto obtain the signal value representing the sum of the signals from thepair of photoelectric conversion elements 43 and 44.

(3) Other Embodiments

The image sensors achieved in the embodiments described above includeimage-capturing pixels equipped with color filters disposed in a Bayerarray. However, the color filters may be structured or arrayeddifferently from those described above, and the present invention may beadopted in conjunction with an image sensor having an array patternother than the Bayer array pattern, such as a complementary color filter(green G, yellow Ye, magenta Mg and cyan Cy) array pattern.

Furthermore, the present invention may be adopted in a monochrome imagesensor that does not include color filters.

It is to be noted that the present invention may be adopted in animage-capturing device other than a digital still camera, such as thatdescribed above, or a film-type still camera used in conjunction withinterchangeable lenses that can be attached to the camera body. Forinstance, the present invention may be adopted in a digital still cameraor a video camera with an integrated lens. The present invention may befurther adopted in a compact camera module included as a built-in unitin a portable telephone or the like, a surveillance camera, a visualrecognition device in a robot, an on-vehicle camera and the like.

The above described embodiments are examples, and various modificationscan be made without departing from the scope of the invention.

What is claimed is:
 1. A backside illumination image sensor thatincludes a semiconductor substrate with a plurality of pairs ofphotoelectric conversion elements formed thereat and a read circuitformed on a side where a front surface of the semiconductor substrate ispresent, and captures an image by outputting, via the read circuit,electrical signals generated as incident light having passed through animage forming optical system and having reached a side where a backsurface of the semiconductor substrate is present, is received at aplurality of pairs of light receiving areas, each pair of lightreceiving areas being formed in each pair of the plurality of pairs ofphotoelectric conversion elements, comprising: an on-chip lens formed ata position set apart from the each pair of light receiving areas by apredetermined distance; a first separating area that is constituted witha type of semiconductor and that separates unit pixel areas from eachother, each of the unit pixel areas including the each pair of theplurality of pairs of photoelectric conversion elements; and a secondseparating area that is constituted with the type of semiconductor andthat forms a boundary between the each pair of photoelectric conversionelements; wherein: the each pair of light receiving areas and an exitpupil plane at which an exit pupil of the image forming optical systemis present achieve a conjugate relation to each other with regard to theon-chip lens.
 2. A backside illumination image sensor that includes asemiconductor substrate with a plurality of pairs of photoelectricconversion elements formed thereat and a read circuit formed on a sidewhere a front surface of the semiconductor substrate is present, andcaptures an image by outputting, via the read circuit, electricalsignals generated as incident light having passed through an imageforming optical system and having reached a side where a back surface ofthe semiconductor substrate is present, is received at a plurality ofpairs of light receiving areas, each pair of light receiving areas beingformed in each pair of the plurality of pairs of photoelectricconversion elements, comprising: an on-chip lens formed at a positionset apart from the each pair of light receiving areas by a predetermineddistance, wherein: the each pair of light receiving areas and an exitpupil plane at which an exit pupil of the image forming optical systemis present achieve a conjugate relation to each other with regard to theon-chip lens; and a radius of curvature R of the on-chip lens, adistance D from an apex of the on-chip lens to the each pair of lightreceiving areas and an average refractive index n of a medium presentbetween the on-chip lens and the each pair of light receiving areasachieve a relationship expressed as: D=R·n/(n−1).
 3. A backsideillumination image sensor according to claim 2, wherein: the pluralityof pairs of photoelectric conversion elements are disposed in atwo-dimensional grid array; and a pitch P of the two-dimensional gridarray, a smallest F number F0 of the exit pupil of the image formingoptical system and the distance D achieve a relationship expressed as:F0·P·n>D>P·n/(2·(n−1)).
 4. A backside illumination image sensoraccording to claim 3, wherein: the radius of curvature over a peripheryof the on-chip lens is greater than the radius of curvature at a centralarea of the on-chip lens.
 5. A backside illumination image sensoraccording to claim 2, further comprising: a light shielding member thatprevents entry of passing light, which is part of the incident lightthat passes through the on-chip lens, into a pair of photoelectricconversion elements corresponding to an adjacent on-chip lens adjacentto the on-chip lens, among the plurality of pairs of photoelectricconversion elements, wherein: the light shielding member is disposedbetween the on-chip lens and the each pair of light receiving areas, thelight shielding member includes a first color filter disposed near theon-chip lens and a second color filter assuming a color matching thecolor of the first color filter, which is disposed near the each pair oflight receiving areas; and a color of the light shielding member isdifferent from a color of a light shielding member disposed inconjunction with the adjacent on-chip lens adjacent to the on-chip lens.6. A backside illumination image sensor that includes a semiconductorsubstrate with a plurality of pairs of photoelectric conversion elementsformed thereat and a read circuit formed on a side where a front surfaceof the semiconductor substrate is present, and captures an image byoutputting, via the read circuit, electrical signals generated asincident light having passed through an image forming optical system andhaving reached a side where a back surface of the semiconductorsubstrate is present, is received at a plurality of pairs of lightreceiving areas, each pair of light receiving areas being formed in eachpair of the plurality of pairs of photoelectric conversion elements,comprising: an on-chip lens formed at a position set apart from the eachpair of light receiving areas by a predetermined distance; and a barriermember that is disposed parallel to an optical axis of the on-chip lensand that prevents entry of passing light, which is part of the incidentlight that passes through the on-chip lens, into a pair of photoelectricconversion elements corresponding to an adjacent on-chip lens adjacentto the on-chip lens, among the plurality of pairs of photoelectricconversion elements, wherein: the each pair of light receiving areas andan exit pupil plane at which an exit pupil of the image forming opticalsystem is present achieve a conjugate relation to each other with regardto the on-chip lens the barrier member is disposed between the on-chiplens and the each pair of light receiving areas; and an anti-reflectionfilm is formed at a surface of the barrier member so as to preventreflection of the passing light.
 7. A backside illumination imagesensor, comprising: a plurality of pairs of photoelectric conversionelements each including a pair of light receiving areas that receiveincident light having entered a back surface of a semiconductorsubstrate through an image forming optical system, a read circuit beingformed on a front surface of the semiconductor substrate; an on-chiplens formed at a position set apart from the pair of light receivingareas; a first separating area that is constituted with a type ofsemiconductor and that separates unit pixel areas from each other, eachof the unit pixel areas including each pair of photoelectric conversionelements; and a second separating area that is constituted with the typeof semiconductor and that forms a boundary between the each pair ofphotoelectric conversion elements, wherein: the pair of light receivingareas and an exit pupil of the image forming optical system achieve aconjugate relation to each other with regard to the on-chip lens.
 8. Abackside illumination image sensor, comprising: a plurality of pairs ofphotoelectric conversion elements each including a pair of lightreceiving areas that receive incident light having entered a backsurface of a semiconductor substrate through an image forming opticalsystem, a read circuit being formed on a front surface of thesemiconductor substrate; an on-chip lens formed at a position set apartfrom the pair of light receiving areas by a predetermined distance,wherein: the pair of light receiving areas and an exit pupil of theimage forming optical system achieve a conjugate relation to each otherwith regard to the on-chip lens; and a radius of curvature R of theon-chip lens, a distance D from an apex of the on-chip lens to the pairof light receiving areas and an average refractive index n of a mediumpresent between the on-chip lens and the pair of light receiving areasachieve a a relationship expressed as: D=R·n/(n−1).
 9. A backsideillumination sensor, comprising: a plurality of pairs of photoelectricconversion elements each including a pair of light receiving areas thatreceive incident light having entered a back surface of a semiconductorsubstrate through an image forming optical system, a read circuit beingformed on a front surface of the semiconductor substrate; an on-chiplens formed at a position set apart from the pair of light receivingareas; and a barrier member that is disposed parallel to an optical axisof the on-chip lens and that prevents entry of passing light, which ispart of the incident light that passes through the on-chip lens, into apair of photoelectric conversion elements corresponding to an adjacenton-chip lens adjacent to the on-chip lens, among the plurality of pairsof photoelectric conversion elements, wherein: the pair of lightreceiving areas and an exit pupil of the image forming optical systemachieve a conjugate relation to each other with regard to the on-chiplens the barrier member is disposed between the on-chip lens and thepair of light receiving areas; and an anti-reflection film is formed ata surface of the barrier member so as to prevent reflection of thepassing light.